Spray Foam Insulation - Roof Undercoating Advice

Sand-lime mortar reinforced with animal hair was applied to the backs of tiles and slates for hundreds of years before roofing underlay was invented. This method, known as torching, was developed to improve weatherproofing and to help prevent tiles and slates blowing off. This practice has all but disappeared, with the exception of the conservation of listed buildings.

Today, a range of polyurethane and PVC foams are available that act in a similar way and add additional insulation to the home.

Whilst the foam can improve the thermal efficiency of your roof, there is growing evidence that under-coatings increase the cost of future re-roofing as the foam bonds tightly with the tiles and is difficult to remove. It also makes it very unlikely that any roof tiles or slates could be salvaged, which further adds to the future re-roof cost.

Another disadvantage that is worth noting is that any damaged or missing tiles or slates will need replacing and correctly fixing prior to the under coating being applied. The same applies to roof flashings and junctions that may be failing. These repairs can only be done from the outside of the roof and will most probably require scaffolding. This can significantly push up the overall price of the work, since scaffolding can represent 1/3 of the cost of a completely new roof.

You will then need to carefully consider whether it is worth paying to erect scaffold, especially for an older roof that will require new tiles or slates some at point in the future, and then paying to install a foam backing which will add to your inevitable re-roof costs further down the line.

If you are interested in spray foam insulation, ensure you: -

Check the system or material has independent technical approval, such as a BBA Certificate.

Ensure your quote includes an external survey and any necessary external repairs using the appropriate scaffold access.

Compare the total cost of the above with a full roof replacement. When doing this consider how much longer your current roof covering will last and factor in the cost of a roof replacement in the future

Please note - spray foam insulation is only really applicable to older roofs that do not have any underlay. It can prevent ventilation of the batten cavity, lock in moisture and damage roof tiles and slates.